1. Field of the Invention
The present invention relates to a hydraulic shock absorber for vehicles and the like.
2. Description of the Prior Art
A conventional hydraulic shock absorber comprises a cylinder, a piston slidably fitted in the cylinder, dividing the interior of the cylinder into two cylinder chambers and provided with a plurality of through holes to allow the two cylinder chambers to communicate with each other, and a disk valve assembly having a plurality of superposed disk valve elements and provided at the outlet end of the corresponding through hole of the piston. The disk valve assembly controls the flow of the working fluid from the cylinder chamber on the inlet side of the through hole into the cylinder chamber on the outlet side of the through hole to generate a damping force when the piston is moved axially within the cylinder.
Japanese Utility Model Public Disclosure (Kokai) No. 61-40533 discloses such a hydraulic shock absorber as shown in FIGS. 7 to 10. This known hydraulic shock absorber has a cylinder 2, a piston 1 axially slidably fitted in the cylinder 2, dividing the interior of the cylinder 2 into an upper cylinder chamber 3 and a lower cylinder chamber 4 and provided with extension side passages 5 and contraction side passages 6, and a first valve assembly 7 and a second valve assembly 8 provided, respectively, at the outlet ends of the extension side passages 5 and the contraction side passages 6. The first valve assembly 7 is constructed by contiguously superposing a large disk valve element 7A, rings 7B, and smaller disk valve elements 7C. The second valve assembly 8 is constructed by contiguously superposing a larger disk valve element 8A, a ring 8B and smaller disk valve elements 8C. The rings 7B and 8B are disposed around the smaller disc valve elements 7C and 8C, respectively, and are biased toward the piston 1 by the larger disc valve elements 7A and 8A, respectively.
Chambers 9 and 10 are formed, respectively, in the opposite ends of the piston 1 so as to be connected, respectively, to the extension side passages 5 and the contraction side passages 6. The piston 1 is provided also with outer chambers 13 and 14 in its opposite ends, respectively, radially outside the chambers 9 and 10, and restricting passages 11 and 12 interconnecting the chambers 9 and 10, and the outer chambers 13 and 14, respectively.
When the piston 1 moves axially to the left (the right) as viewed in FIG. 7, within the cylinder 2 for the extension (contraction) stroke of the hydraulic shock absorber, the larger disk valve element 7A (larger disk valve element 8A) and the rings 7B (the ring 8B) of the first valve assembly 7 (the second valve assembly 8) are deflected to open by the pressure difference between the upper cylinder chamber 3 and the lower cylinder chamber 4 to allow the working fluid to flow through the extension side passages 5 (the contraction side passages 6), the chamber 9 (the chamber 10), the restricting passage 11 (the restricting passage 12) and the outer chamber 13 (the outer chamber 14) from the upper cylinder chamber 3 (the lower cylinder chamber 4) into the lower cylinder chamber 4 (the upper cylinder chamber 3). As the pressure difference increases, namely, as the moving speed of the piston 1 increases, the smaller disk valve elements 7C (the smaller disk valve elements 8C) are separated from the corresponding end surface of the piston 1 to allow the working fluid to flow through the extension side passages 5 (the contraction passages 6) and directly through the chamber 9 (the chamber 10) from the upper cylinder chamber 3 (the lower cylinder chamber 4) into the lower cylinder chamber 4 (the upper cylinder chamber 3). The hydraulic shock absorber thus exerts its damping force characteristics according to the pressure difference between the upper cylinder chamber 3 and the lower cylinder chamber 4.
However, a conventional shock absorber of such a type having a comparatively small effective diameter is provided with a piston having a comparatively small diameter and hence has difficulty in forming an extension side passage and a contraction side passage each having a sufficiently large sectional area through the piston. The respective diameters of the passages 5 and 6, hence the respective sectional areas of the passages 5 and 6, of the conventional hydraulic shock absorber having the foregoing construction, in particular, are limited, respectively, by the widths of the chambers 9 and 10, because the outer chambers 13 and 14 are disposed, respectively, radially outside the chambers 9 and 10. When the sectional areas of the passages 5 and 6 are not sufficiently large, the working fluid is unable to flow from the upper cylinder chamber 3 into the lower cylinder chamber 4 and vice versa, particularly, from the lower cylinder chamber 4 into the upper cylinder chamber 3 for the contraction stroke in which a comparatively small damping force is desirable, because the contraction side passages 6 having a comparatively small diameter act as orifices and restrict the flow of the working fluid during the opening action of the smaller disk valve elements 8C, so that the hydraulic shock absorber is unable to exert its desirable damping force characteristics for the contraction stroke. An increase in the number of passages to increase the total sectional area of the passages entails increase in time required for boring the passages because the passages must be formed obliquely with respect to the axis of the piston, and increase in the number of passages on a circle reduces the strength of the piston. Accordingly, it has been desired to increase the total sectional area of the passages and to form the passages in an optimum arrangement without reducing the strength of the piston and without requiring difficult machining operations.